The invention relates to a control device for the intermittent change-over of a vehicle drive from two-wheel drive via a permanently driven vehicle axle to four-wheel drive. This change-over taking place as a result of the closing of an electrically or electrohydraulically controllable clutch which, in the closed state, couples the permanently active drive train in drive terms to the connectable drive train acting on the further vehicle wheels. This change-over is triggered when the drive slip of at least one of the wheels of the permanently driven vehicle axle exceeds a predetermined threshold value. An electronic control unit generates control signals necessary for controlling the clutch as a result of a processing of electrical output signals from speed sensors characteristic of the circumferential speeds of the wheels belonging to the respective vehicle axles.
A control device of this type is shown in German Offenlegungsschrift No. 3,427,725 for a vehicle with a permanently active front-axle drive train and a connectable rear-axle drive train. For coupling the rear axle-drive train in drive terms to the power take-off gear or the front-axle drive train, there is a slipping clutch of controllable closing force, which is controlled of the electronic control unit in such a way that the slip of the permanently driven vehicle wheels is always a predeterminable amount greater than the slip of the vehicle wheels driven by the connectable drive train.
Disadvantages of this known control device are, on the one hand, the high wear occurring in the region of the slipping clutch and, on the other hand, to be considered more critical, the loss of driving stability resulting from the fact that the permanently driven vehicle axle is set to a higher drive slip in relation to the connectable driving axle. This is a disadvantage particularly when the permanently active drive train is the rear-axle drive train.
This last-mentioned disadvantage also plagues a purely hydraulically controlled device acting in a similar way, as shown in from German Offenlegungsschrift No. 2,805,692 for a vehicle with a permanent rear-axle drive and intended for connecting a front-axle drive train to a permanently active rear-axle drive train. In this device, the change-over to four-wheel drive likewise taking place when the drive slip of the permanently driven rear wheels exceeds a specific threshold value. The two drive trains are coupled for four-wheel operation by a pressure-controlled multiple-disc clutch which, although it can "play" between the closed and released operating states during forward motion, nevertheless has to remain closed for reversing. In this case, during cornering, considerable stresses can arise once again in the drive train because of the different cornering radii for the steerable front wheels and the rear wheels of the vehicle. In extreme cases, particularly when the cornering radii are narrow and there are high adhesion coefficients between the vehicle wheels and the road, these stresses can lead to considerable distortion in the drive train as a whole and consequently result in damage to the drive train.
The object of the invention is, therefore, to provide an improved control device such that, in four-wheel operation, both good driving stability and effective protection of the drive train against distortion stresses during cornering are obtained.
According to the invention, this object is achieved by providing a torque direction sensor which provides logic output signals for the two possible alternative directions of torque in effect in the connectable drive train and a driving direction sensor for providing alternative logic level output signals for the alternative forward and reverse driving directions. The electronic control for the valve controlling the clutch which connects and disconnects the connectable drive train receives signals from the torque direction sensor and the driving direction sensor and the wheels to control the clutch as follows. The electrical control unit generates a signal for a minimum period of activation to close the clutch and connect the connectable drive train when the slip threshold for at least one of the wheels of the permanently driven vehicle axle exceeds a predetermined threshold. The control electronics further controls the opening or closing of the clutch depending on the signals from the torque direction sensor and the driving direction sensor as will be more fully explained below.
The control device according to the invention provides the following functional properties and benefits, as explained by the example of a vehicle with a permanent rear-axle drive:
A signal causing the front-axle drive train to be connected to the permanent rear-axle drive train, after being triggered because a drive-slip threshold on the rear axle has been exceeds, is maintained for a minimum period of time which amounts to at least a few seconds, for example 3 to 10 seconds. This period is very much greater than the minimum periods of a few 100 milliseconds which are necessary for adjusting and monitoring slip values and in which the particular state of motion of the vehicle wheels could be checked. Consequently, if appropriate, a change back to two-wheel drive could be made. Lengthening the periods of time in which the additional drive train is connected makes it possible to check the condition of the road. Thus, the periods in which a drive slip impairing driving stability could build up on the permanently driven rear axle, are drastically reduced, thereby achieving altogether a considerable gain in terms of driving stability.
The connectable front-axle drive train is disconnected within the longer period of time only when and as long as an output signal from the torque direction sensor of the front-axle drive train indicates that the torque in the connected drive train has a direction, representing a deceleration of the vehicle, as seen in the driving direction. Such signals are generated by the torque direction sensor, when, during forward cornering, an accelerating torque acts on the front wheels as a result of the adhesion between the road and the front wheels and the larger radii of their tread paths. But because of the said adhesion the front wheels cannot follow this accelerating torque or can only follow it insufficiently. Thus, a pulling moment builds up in the front-axle drive train. Such signals are also generated when, during cornering in reverse, a pushing moment occurs in the front-axle drive train for the reasons mentioned. For the length of time such moments occur, the clutch is then released within the predetermined connection period, and it thereby becomes possible to reduce the moments which would otherwise lead to dangerous stresses in the drive train. In cornering situations, during which, within the connection period, the vehicle runs over regions of the road with sharply changing adhesion coefficients, the best possible compromise between driving stability and the protection of the drive train as a whole is achieved. In a cornering situation, in which, after the front-axle drive train has initially been connected, the vehicle runs over regions of the road with a uniformly high adhesion coefficient during a further portion of the connection period, torsional stresses which would otherwise occur are reduced according to need.
The torque direction sensor which can be used within the framework of the control device according to the invention includes a three by three way valve with a middle and two alternate flow through positions which is positioned as a result of the alternative directions of torque effective in the connectable drive train. In the middle position of the valve corresponding to a torque-free state of the connectable drive train, the two pressure outlets are cut off from the control pressure source. In the two flow through positions, one of the two outlets is connected to the inlet pressure and the other is shut off. If there is already a hydraulic auxiliary-pressure source on the vehicle it is possible to produce this torque direction sensor by means of a simple hydraulic directional valve and a pressure/voltage converter device consisting, for example, of simple pressure switches.
The three/three-way valve is a rotary-slide valve including a piston supported on the output gear by at least one resilient or torsional element for restoring force increasing counter to the relative movement of the piston and the output gear of the power take off gear. Stops are provided to define a limited angular sector defining the positions of the middle and the two alternating positions. This provides a space-saving, simple and functionally reliable design of the valve of the torque direction sensor, and this can be incorporated in the power take-off gear. By making the torsional rod one piece with the piston and connected fixedly in terms of rotation to the output gear wheel and by projecting into the bore located in the outside shaft piece of the output gear wheel.
The pressure/voltage converter device is designed especially favorably in functional terms as a flow-resistance measuring bridge with preferably symmetrical parallel flow branches and with a simple differential-pressure switch as a signal transmitter. The output signals of the differential pressure switch are combined in a simple logical circuit with the forward/reverse motion signals of the driving-direction sensor, to form the signals for the appropriate activation of an electrohydraulically controlled clutch, which includes a solenoid fluid valve connecting a fluid pressure source with a fluid clutch.
The control device according to the invention is equally suitable for vehicles with a permanent rear-axle drive and a connectable front-axle drive and for vehicles with a permanent front-axle drive and connectable rear-axle drive.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.